In the plastics industry, screws and cylinders are used in various injection molding and extrusion processes. This invention relates to a corrosion and wear resistant bimetallic cylinder used in these processes.
2. Description of the Related Art
In injection molding and extrusion processes, solid plastic resin is heated and liquefied inside a hollow cylinder by heater bands that envelop the cylinder and by the frictional heat generated by the rotational action of the screw. The molten plastic is discharged from the cylinder and enters a mold. The molten plastic solidifies in the mold to form various objects
During the plasticization process, the plastic resin will wear against the bore of the cylinder. Besides, the plastic often contains various fillers which are highly abrasive Also, aggressive chemicals generated during the process can corrode the bore. For instance, fluoropolymer, polyvinyl chloride, and resins containing flame retardants will generate hydrofluoric or hydrochloric acid, which are very corrosive. If the bore of the cylinder is enlarged due to wear or corrosion, the efficiency of the process will decline.
One approach to solve the problem of wear is to nitride the steel bore The nitrided surface is very hard and resistant to wear. However, the nitrided layer is only 0.005 to 0.020 inches deep, and the concentration of nitride decreases rapidly with the depth of the layer. The wear resistance of the nitrided surface will drop quickly with time as the very hard top surface of the nitride layer is worn. Furthermore, such a cylinder has a poor corrosion resistance.
A second approach is to cast a hard and wear resistant alloy inside a hollow steel cylinder by a centrifugal casting process. The melting point of the casting alloy is lower then that of the steel by a few hundred degrees Fahrenheit. In this process, the alloy is loaded inside the hollow cylinder, and the ends of the cylinder are sealed. The cylinder is then placed in a furnace at a temperature high enough to melt the alloy but sufficiently lower than the melting point of the steel. The steel cylinder is then rotated rapidly about its axis to distribute the molten alloy as a continuous layer on the inside of the cylinder. Upon cooling, this molten alloy solidifies and metallurgically bonds to the steel bore to form a hard, and wear resistant inlay. This inlay then is honed to the correct diameter and surface finish. The inlay is typically 0.060 inches to 0.125 inches thick. The chemical composition and hardness is uniform across the entire inlay thickness. The inlay alloy contains various corrosion resistant elements. Presently, such a bimetallic cylinder is most reliable to resist corrosion and wear in all injection and extrusion processes.
Facing the increasing demand of processing more abrasive and corrosive resins, inlay alloy design for a bimetallic cylinder is very critical. The following must be satisfied to make such a cylinder:
1. The melting point of the inlay must be a few hundred degrees Fahrenheit below the melting point of the steel.
2. The inlay alloy must have good castability so that the inlay will not have any casting defects such as shrinkage voids or porosity
3. The inlay alloy must have good hardness and wear resistance.
4. The inlay alloy must have good corrosion resistance.
5. The inlay alloy must have reasonably good resistance to thermal shock, or the inlay will crack upon cooling during fabrication.
6. The inlay alloy must have sufficient fracture strength or the cylinder will not have a sufficient pressure carrying capacity.
It is an object of the invention to provide a bimetallic cylinder which avoids the need for a nitrided layer and has good corrosion and wear or abrasion resistance.
It is another object of the invention to provide a bimetallic cylinder having an alloy inlay that achieves at least a 40% improved wear or abrasion resistance than standard alloy inlays.
A further object of the invention is to provide an alloy inlay having an acceptable level of corrosive wear resistance in normal processing environments including the processing of low and high density polyethylene, polyvinyl chloride and acrylonitrile-butadiene-styrene copolymer.
A further object of the invention is to provide an inexpensive alloy inlay for bimetallic cylinders for use in place of premium alloy inlays.
A further object of the invention is to provide an alloy inlay that provides adhesive wear compatibility with normal screw hardfacing alloys, i.e., stellite 6 and 12, colmonoy 56, nitralloy 135 nit , and flame hardened screws.
A further object of the invention is to provide an alloy inlay with improved pressure carrying capacity in the "as cast" condition, which is important in injection molding equipment.